The present invention relates to securing the operation of voltage holding devices for on-board networks of vehicles, especially automobiles.
Electrical energy consuming accessories have a tendency of becoming more numerous in automotive vehicles. Examples of such accessories include air conditioners or power steering systems or devices for automatic stop and start of combustion engines, systems which are currently called “Stop & Start”. For instance, Stop & Start systems can comprise a reversible alternator, or “alternator-starter”, for restarting the combustion engine and recuperating kinetic energy when the vehicle decelerates and for transforming this energy into electrical energy used to recharge the battery.
These kind of devices, which are used to restart the combustion engine, have high energy demands, require significant electrical power, and in turn cause high electrical currents. This energy demand must be satisfied without over-sizing the electrical supply means and without provoking a voltage drop in the on-board network, which would result in a perception of poor vehicle quality and a temporary reduction of the engine torque.
To resolve these problems, it has been proposed to add an auxiliary energy source, such as a second battery connected to the on-board network. This solution however is relatively costly and requires space under the hood or in the trunk of the vehicle.
Another solution consists in adding to the network secondary means for storing electrical energy, such as high performance capacitors (for instance 300 F capacitors). This solution can be used in addition to the traditional 12V battery for starting and restarting and for powering the electricity-consuming accessories connected to the network. The high capacitance of these capacitors however causes difficult to avoid inconveniences, for instance Joule effect heating of the electrical conductors.
Patent application FR 2 853 154 filed on Mar. 27, 2003 proposes a “DMT” type (Voltage Holding Device or “DMTC” Centralized Voltage Holding Device) supply system of electrical energy for stabilizing the voltage of the on-board network. These electrical voltage holding devices appear to be almost indispensable when the vehicle is equipped with a Stop & Start system.
The voltage holding device, used during the restart phases of the combustion engine, must be activated and deactivated rapidly in order to avoid voltage surges or drops in the on-board network. For instance, the overvoltage provided by the secondary energy storage means, such as super-capacitors, at the moment the engine is restarted, can be in the order of 4 to 5 Volts in addition to the 12V supplied by the battery. Of course, this overvoltage must be suppressed rapidly when the restart phase is terminated, at the risk of damaging electrical devices that do not tolerate a voltage on the order of 12 Volt plus 4 or 5 Volt. Therefore, the voltage holding device must contribute energy only when necessary.